(1) Field of the Invention
The present invention relates to semi-fuel cells, and more specifically to a new treatment for ion exchange membranes that accelerates the wetting of the membranes by aqueous electrolyte solutions, thus reducing the start up time for metal/hydrogen peroxide-based semi-fuel cells.
(2) Description of the Prior Art
A semi-fuel cell is essentially a hybrid of fuel cells and batteries. Combining the refillable cathode or catholyte oxidizer of fuel cells with the consumable anode fuel of batteries. Semi-fuel cells are currently under investigation as electrochemical power sources for unmanned undersea vehicles. In a semi-fuel cell, a metal anode, such as magnesium or aluminum along with a liquid catholyte, typically a strong oxidizer like hydrogen peroxide, are consumed to produce energy. The electrochemical reaction is given below for magnesium with hydrogen peroxide in acid media.
Anode:Mg −> Mg2+ + 2e−2.37 vCathode:H2O2 + 2H+ + 2e−2H2O1.77 vCell Reaction:Mg + H2O2 + 2H+Mg2+ + 2H2O4.14 v
In addition to the primary electrochemical reaction, several parasitic reactions can also take place.Decomposition: 2H2O2→2H2O+O2 Direct Reaction Mg+H2O2+OH−→Mg2++3OH−Corrosion: Mg+2H2O→Mg2++2OH−+H2 
Of the three parasitic reactions listed above, the direct reaction is the most detrimental to the operation of the semi-fuel cell since both magnesium and hydrogen peroxide are consumed in a single step. Whereas magnesium corrosion can be suppressed by pH adjustment and hydrogen peroxide decomposition minimized by careful temperature control, prevention of the direct reaction requires that the magnesium anode and hydrogen peroxide catholyte be physically separated from each other. To accomplish this, a semi-permeable membrane capable of ion exchange is placed between the anode and cathode compartments of the semi-fuel cell in order to isolate the anolyte and catholyte solutions.
In order for ionic transport to occur across a membrane, the membrane must first absorb the electrolyte solution. The membrane's rate of absorption determines how quickly the semi-fuel cell reaches its operating voltage. In most applications, semi-fuel cells are stored “dry” to prevent corrosion of the magnesium anode. When electrical energy is needed, the semi-fuel cell's anode and cathode compartments are flooded with electrolyte so that power generation can begin. The membrane must then wet immediately upon contact with the electrolyte, so that the semi-fuel cell can begin to supply power. In the situation where the semi-fuel cell is being used with an unmanned underwater vehicle, the requirement for power generation is within seconds of the vehicle's deployment. Ion exchange membranes, such as NAFION®, require a long pre-soak period (at least 12 hours) in aqueous electrolyte solution in order to be fully wet and therefore fully functional in a semi-fuel cell. A method to accelerate the wetting of the membrane is needed.
Several prior art methods exist for treating a NAFION® membrane to make its performance more consistent and reproducible. However, these techniques are designed to remove impurities from the manufacturing process and/or exposure to the environment rather than enhance the membrane's ability to absorb liquid. Typical treatments involve boiling the membrane in dilute acid followed by rinsing in boiling distilled water. After treatment, the membrane must remain wet prior to use.
What is needed is a method that will allow an ion exchange membrane such as a NAFION® membrane in a semi-fuel cell to be stored dry and then to wet immediately when the semi-fuel cell is activated.